A 'serial house mover', interior designer and former actress Trevyn McDowell has done rather better treading the decking than she did treading the boards. Born in South Africa, she came to London at 16 to study drama for a year. Twenty years later, she is about to return. She is married to theatre designer Julian McGowan and lives in a converted office building in Clerkenwell

I started when I was 16, buying a little flat in West Hampstead with the money I had as a child actress. I sold it four years later for 100 per cent more than I paid for it. Then I bought a maisonette in Belsize Park, followed by a Georgian house full of antiques in Chiswick.

"When I bought this house from a developer as a shell, I was single with no children and it was fantastic for parties. I just had to have this space; light streams in from everywhere. I borrowed and applied for the money to buy it all over the place. I am a risk taker. At that point, people didn't lend on shells. I had to put in a kitchen and bathroom and some plumbing before they'd lend me the money. Because I got in quite early, the architects kmk and I did our own scheme. There was a flat on the ground floor, but I talked the owner into selling it to me and now it's our office.

"I decided to stop being an actress when I started my company, Site Specific. I felt frustrated at the lack of control you have as an actress, unless you are very successful. Now I am less neurotic as a person. Fitting out warehouse conversions started as favours for friends, then it snowballed. It just took on a life of its own and now there are seven of us working here. We do everything from finding a property to getting planning permission, right down to buying the teaspoons if that's what people want.

"We've worked a lot for people who have gone away filming and they'd leave the project with us and trust us to get on with it. We have 10 to 15 projects on at any one time. Having been an actress, I tend to get a lot of actor clients. But my family are all interior designers, so I've grown up with it. I'm very hands-on. The staircase is steel and I angle-grinded it myself. It has clip-on panels which can fix onto the sides, to stop children falling through. They can just be packed away when not needed.

"It was my idea to have the folding doors so you can open up the bedroom to the terrace and the rest of the room, or you close it up for privacy. The glass for the atrium had to be lifted in by crane onto the roof. The CD racks and book shelves were all custom-built, with opaque glass sliding panels to hide things away. We used the same glass for the windows which face other buildings.

"Is it impossible to keep tidy? Oh no. The kids' toys - we have Ruby who is two and a half, Zachary is a year old and Julian's son Jacob who is nine - are all kept in the cupboards. We keep most of the toys in the garage, so we rotate them. I find great serenity in being tidy. I'm good at throwing things out, I'm very disciplined.

"We have rented the house out for photo shoots and commercials and it's been a useful source of income. That helps you get into the habit of keeping things at an certain level of tidiness. We stopped doing the location shoots when we had the children. But the first time we did it, we went off and stayed at the St Martin's Hotel for a week. It was a lovely start to a relationship.

"The house is so big, we can be working downstairs while there are 80 people up here shooting and you can't even hear them. Julian and I were married in the house in 1996. We had 200 guests in here and you could see from the pictures that it doesn't look crowded. We've also had friends' gallery launches here. The space here is fantastic and with all the outside space, it gives a feeling of inside/outside. The main deck feels very South African, or Australian, with the olive trees, the agapanthus and water feature that I designed. Ruby is old enough to know not to go near the water, but Zachary did fall in once - head first, with feet in the air.

"We've now bought a little house on the beach in Wilderness, four hours' drive along the Garden Route from Cape Town in South Africa. I'd like the children to have an outside life, but we'll keep a home here in London. I will have to give up the luxury of working where I live. We are enlarging the Wilderness house, but want it to remain organic, built into the sand dunes and cutting into rock. Some of the big homes there are very Miami Vice style, but we want to be more invisible. It's been great fun designing a house together. I can't think of any better place to bring up kids than on the beach, with dolphins in the bay."

Trevyn McDowell's house in Clerkenwell, London EC1, is for sale at £2.25m through Hurford Salvi Carr on 020-7250 1012